In around the year 100 BC, Hero of Alexandria discovered the reaction engine with his aeolipile experiment. Then the Wright brothers introduced powered flight in 1903, and in 1910 Henri Marie Coanda married the two together to fly the first jet-propelled airplane. In 1955 the McDonnell XV-1 tip-jet autogyro made its record-breaking flight.
With man's desire for aircraft that would use less runway coincident with flying faster, safer and easier handling, he has created many different types of VTOL, SVTOL or compound aircraft including helicopters, autogyros, tilt-rotors, tip-jets, convertiplanes, gyrodynes, contra-rotators and direct lift Harrier types.
The field of compound aircraft can be broken down into three broad categories relative to their rotors:                4. Shaft driven rotors, which employ the use of high-maintenance mechanical drive systems, are often massive, complex, expensive, and inefficient. They cause accompanying aircraft to be expensive, have poor economy of operation and maintenance, poor empty-weight fraction, poor range, poor performance, and still other factors that will be discussed in more detail below.        Typical of the shaft driven compound aircraft is the aircraft disclosed in U.S. Pat. No. 3,155,341, by P. F. Girard. Another is disclosed in U.S. Pat. No. 5,174,523, which discusses “engine shaft power output control.” Both include heavy, expensive gearboxes and attendant shafting. Both have fixed wings, but neither speaks of providing any means of transiting the mu-1 barrier. (The term mu is the ratio of the forward velocity of the aircraft relative to the tip velocity of the rotor.) Another related U.S. Pat. No. 4,730,795 describes a heavy, complex co-axial shaft transmission that drives gyrating blades. Another of the shaft-driven type is the Lockheed AH-56A Cheyenne. These shaft-driven aircraft generally require an anti-torque device.        5. Autogyro and gyroplane type aircraft normally lack means of rotor drive while airborne. They usually require a short takeoff or a jump takeoff if the rotor is powered up on the ground. The big disadvantages of the autogyro types are the lack of hovering ability, agility, responsiveness, room for error, and limited freedom of choice in VTOL maneuvers. This group is tricky and unforgiving to land, as rotor rpm can be difficult to manage. U.S. Pat. No. 5,727,754, discusses a gyroplane commonly known as the CarterCopter, which due to blade weights can store enough kinetic energy in the rotor to enable the craft to make a fifty-foot high jump takeoff.        Another recent U.S. Pat. No. 6,513,752, discusses an aircraft that is classified as a gyro-type of aircraft. The aircraft has a transmission, clutch and drive train to power the rotor when needed. This adds more weight and expense to the aircraft, and, in addition, necessitates an anti-torque device.        6. A third category includes VTOL aircraft capable of flying vertically via gas-powered tip-jet-driven rotors. The McDonnell XV-1 has a tip-jet driven rotor, and although it operated as an autogyro in forward flight, it had exceeded contemporary rotary-wing speed records by hitting 200 mph. It did not have the capability to exceed mu-1, however. This type of craft does not require an anti-torque device. These aircraft have the advantages of a powered rotor but need not have a heavy, massive drive train, transmission, clutch, rotor head and blade root sections.        
We will confine our further remarks here to category 3 above: VTOL aircraft capable of flying vertically via gas-powered tip-jet-driven rotors.
U.S. Pat. No. 4,589,611 discloses a contra-rotating aircraft with a massive rotor head and extra lifting hardware to slow the aircraft. The lifting hardware makes the aircraft inefficient in forward flight.
U.S. Pat. No. 3,635,426 discloses an aircraft that employs high rotor rpm during high-speed flight. With no fixed wing, this aircraft's forward speed is limited by advancing tip speed and/or retreating blade stall.
U.S. Pat. No. 5,984,635 discloses an aircraft with at least six blades around the periphery of a large circular plenum that acts as a wing in forward flight. Six or more blades add unnecessary drag in forward flight. The plenum will not be as efficient as a high aspect ratio wing, and it will act as a large reservoir for the compressed gas that will slow the response time when the pilot calls for increased or decreased rotor rpm.
The McDonnell XV-1 employs a conventional helicopter cyclic controlled rotor with all its complexity. The accompanying wing does not totally sustain the aircraft even at top speed. The rotor is propelled by hot tip-jets—that is, fuel is fed to the tip-jets in order to obtain the required thrust. Forward thrust is obtained by the cyclic action of the rotor until the aircraft was largely sustained, but not entirely, by the wing at which time power was transferred from the tip-jet gas compressors to the pusher propeller. In addition, the aircraft flies as an autogyro when in cruise flight. Furthermore, the McDonnell XV-1 derives approximately 15 percent of its lift from the rotor when flying at its maximum speed. It achieves this by putting the rotor in autogyro mode. It did not possess the technology to exceed mu-1, which would have been necessary for it to fly faster than 200 mph. However, it did obtain a mu of 0.95, which has yet to be exceeded.